Recent large increases in demand for nitrogen have been experienced. One primary cause has been enhanced oil recovery by injection of pressurized nitrogen into the well. Production is required on a very large scale and at very high purity (typically less than 5 ppm O.sub.2). Under these conditions, the energy requirement of the producing plant is a major component of the cost of the nitrogen. Accordingly much recent attention has been devoted to lowering the energy required for producing nitrogen.
Prior art patents which disclose reduced energy approaches to dual pressure distillative production of nitrogen include U.S. Pat. Nos. 4,453,957, 4,448,595, 4,439,220, 4,222,756, and British Pat. No. 1,215,377. These all involve supplying feed air to a high pressure rectifier, then routing the rectifier bottom product either directly or indirectly to a low pressure distillation column, and several also involve supplying reboil to the low pressure column by latent heat exchange with vapor from the HP rectifier. They also all incorporate a means of increasing the reflux at the top of the LP column, whereby N.sub.2 purity and yield are increased, by exchanging latent heat between LP column overhead vapor and boiling depressurized LP column bottom product.
The '377 patent was one of the earliest disclosures of the basic configuration described above. It included the option of withdrawing some product N.sub.2 from the HP rectifier overhead, in addition to that withdrawn from the LP column overhead. The '957 patent discloses the same basic configuration, with the modifications of a different method of producing refrigeration and elimination of any transport of liquid N.sub.2 from the HP rectifier overhead to the LP column overhead. The '756 patent also involves the same basic configuration, also eliminates flow of LN.sub.2 from HP rectifier overhead to LP column overhead, and discloses yet another variation for producing refrigeration.
The '220 and '595 patents do not involve reboiling the LP column by latent heat exchange between HP rectifier vapor and LP column liquid. Rather, both of those patents disclose refluxing the HP rectifier by exchanging latent heat with boiling depressurized kettle liquid (HP rectifier bottom product). The at least partially evaporated kettle liquid is then fed into the LP column for further separation. This same technique has been disclosed in processes for producing low purity oxygen, e.g. U.S. Pat. Nos. 4,410,343 and 4,254,629. The latter patent explains by means of a McCabe-Thiele diagram the advantage of this technique--that feeding 40% O.sub.2 vapor to the LP column is more efficient than feeding 40% O.sub.2 liquid to the same column.
The differences between the '220 patent and the '595 patent are that in the '220 patent the LP column is solely a rectifier with no source of reboil other than the vapor feed to it, whereas in the '595 patent the LP column has a stripping section and a reboiler supplied by total condensation of a minor fraction of the feed air. The latter means of reboiling the LP column is also disclosed in the U.S. Pat. No. 4,410,343 for low purity oxygen producing processes.
Reboiling the medium pressure column of a three column triple pressure configuration for producing high purity oxygen by latent heat exchange with partially condensing supply air is disclosed in U.S. Pat. No. 3,688,513. Providing intermediate reboil to a low pressure column by latent heat exchange between HP rectifier overhead vapor and partially evaporating LP column intermediate height liquid is disclosed in U.S. Pat. No. 4,372,765.
The '220 patent has the disadvantage that the N.sub.2 recovery is low. Since the LP column is only a rectifier, the N.sub.2 content of the vapor feed (about 60%) sets a lower limit on the N.sub.2 content of the LP bottom liquid (about 40%), and hence recoveries only on the order of 80% are possible.
The '595 patent has the disadvantage of requiring significantly higher feed pressures than are actually necessary, while achieving lower recoveries than are possible, due to inefficiencies involved in reboiling the LP column by total condensation and in feeding evaporated kettle liquid to the LP column.